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Metamath Proof Explorer

Theorem pm4.65

Description: Theorem *4.65 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

		Ref	Expression
	Assertion	pm4.65	$⊢ \neg (\neg φ \to ψ) \leftrightarrow (\neg φ \land \neg ψ)$

Step	Hyp	Ref	Expression
1		pm4.61	$⊢ \neg (\neg φ \to ψ) \leftrightarrow (\neg φ \land \neg ψ)$